The catalytic mild-hydrocracking process is a petroleum refining operation which converts heavy distillation fractions or residues of cracked petroleum distillates into middle distillates.
In the first step of the reaction, the feedstock, which contains polycyclic aromatics, is solely hydrogenated, after which hydrocracking takes place together with further hydrogenation. In the mild-hydrocracking process that follows, the polycyclic aromatics are hydrocracked to monocyclic aromatics as well as to paraffins. During the mild hydrocracking process, organic nitrogen and sulfur compounds are converted into ammonia and hydrogen sulfide, respectively, to yield sweetened products.
In a typical mild hydrocracking process, the petroleum feedstock is brought into contact with a catalyst which has both a hydrogenation function and an acidic function. The hydrogenation function is provided by a combination of metals such as nickel-tungsten, nickel-molybdenum, cobalt-molybdenum, and the use of noble metals such as platinum, palladium, osmium, etc. The acidic function is provided by the catalytic support or substrate which consists, generally, of alumina modified by additives that improve the surface acidity of the catalyst, such as silicon, phosphorus, boron, fluorine, magnesium, titanium, zirconium, etc.
In order to obtain specific products, a considerable effort has been made to develop and commercialize highly active and selective catalysts for hydroconversion of residues into middle distillates and for the saturation of aromatic compounds contained in these residues.
For example, U.S. Pat. No. 3,016,346 to O'Hara describes a nickel-molybdenum catalyst supported on alumina and a smaller amount of titania (0.5 to 5.0 by weight %). The catalyst contains about 5% to about 10% by weight of molybdenum, about 0.1% to about 0.7% by weight cobalt and about 1% to about 5% by weight nickel, based on the weight of the finished catalyst. The catalyst has hydrodenitrogenation activity and also inhibits those hydrocracking reactions which result in excessive coke deposits on the catalysts when the catalyst is used for hydrotreating a thermally cracked middle fraction of naphtha in a standard relative activity test. The alumina-titania support in O'Hara is prepared by co-precipitation of aluminum and titanium hydroxides from a solution of titanium tetrachloride, an aqueous ammonium hydroxide solution and aluminum chloride and contains 3.7% by weight of titanium.
U.S. Pat. No. 4,465,790 to Quayle discloses a hydrodenitrogenation catalyst comprising catalytic molybdenum and nickel on a co-precipitated support of alumina and titania. The support is co-precipitated from solutions of the sulfates of aluminum and titanium. Titania in the support constitutes more than 5% by weight of the catalyst. The resulting hydrogel is processed using standard techniques to form a catalyst support. Catalytic metals are then impregnated onto the support. The molybdenum loading on the catalyst is between 10% and 25% by weight (as MoO.sub.3) and the nickel loading is from 2% to 10% by weight (as NiO). However, this particular catalyst has relatively poor mechanical properties and is difficult to extrude during manufacture.
European Patent Application 0 199 399 discloses a method for making a hydrocarbon conversion catalyst with improved hydrodesulfurization and hydrodenitrogenation activity. The catalyst contains a Group VI-B metal that is either molybdenum or tungsten and a Group VIII metal that is either cobalt or nickel that are incorporated into a titania-containing alumina support. The catalyst is prepared by mulling together alpha aluminum monohydrate and titanium dioxide at a molar ratio of Al.sub.2 O.sub.3 :TiO.sub.2 ranging from 3:1 to 15:1, acetic acid in a concentration ranging from 2.5 to 3.5 parts of acid per 100 parts of Al.sub.2 O.sub.3 by weight, and sufficient water to produce an extrudable mixture. The mixture is then extruded and calcined. The metals are impregnated onto the support by conventional methods.